Part of the skill of an optician consists in mounting a pair of correcting ophthalmic lenses on an eyeglass frame selected by a wearer. Such mounting comprises three main operations:
acquiring the shape of a surround of the eyeglass frame;
centering the lens, which consists in positioning and in orienting said lens appropriately in front of the corresponding eye of the wearer; and then
machining the lens which consists in cutting away or shaping its outline to the desired shape, given the shape of the surround and the defined centering parameters.
The specific aim of the optician is to shape the ophthalmic lens so that it can be fitted mechanically to the shape of the surround of the selected frame, while ensuring that the lens performs the optical function for which it is designed as well as possible.
With rimmed eyeglass frames, the surround completely surrounds the outline of the lens and is referred to as a rim. Shaping includes a beveling operation that consists in forming all along the edge face of the lens an engagement ridge that is to engage in a groove, commonly referred to as a bezel, running around the inside face of the rim of the eyeglass frame.
With half-rimmed eyeglass frames, the surround goes around only a fraction of the outline of the lens and is referred to as a half-rim. Shaping then comprises a grooving operation consisting in forming along the edge face of the lens an engagement groove, with a fraction thereof being designed to be engaged on a ridge running around the inside face of the half-rim of the eyeglass frame. The lens is held against the half-rim by a string that is connected to the end of the half-rim and that is received in the remaining fraction of the engagement groove.
Generally, in order to ensure that the engagement ridge (or the engagement groove) does not overlie to the front or rear of the edge face of the lens, the optician machines the lens so that the engagement ridge (or the engagement groove) lies substantially halfway across the width of its edge face.
Nevertheless, it is found that once the pair of eyeglasses has been assembled, it does not always present a pleasing appearance, because the positioning of the lenses in the rims of the frame as determined by the above-mentioned technical constraints lead to possible differences in the thicknesses or even in the general curvatures of the two lenses, and to differences in thickness between the edge of the lens and the corresponding rim of the frame. This is particularly widespread since frames come in a very wide variety of shapes.